The cross regulator is provided to effect equitable distribution of supplies amongst the distributary and parent canal, to raise water level when supply.

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Presentation on theme: "The cross regulator is provided to effect equitable distribution of supplies amongst the distributary and parent canal, to raise water level when supply."— Presentation transcript:

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The cross regulator is provided to effect equitable distribution of supplies amongst the distributary and parent canal, to raise water level when supply in the parent canal is low, to release surplus water from canal, in conjunction with escapes, or to provide means for cutting off supplies to the downstream side for repairs etc. The criteria for the Hydraulic Design of cross regulators for canals is as per I.S. code: 7114 – 1973 (reprint December, 1979).

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A hump is provided below the regulator gates creating a fall on the D/S side for the following reasons. (a) To trap silt carried by the water on the U/S side of the regulator. (b) To reduce the depth of flow over the hump to increase velocity through the vents and economize the gate structure. (c) Hump is created in the glacis drops to increase the efficiency of flow of water to D/S side. (d) To negotiate the difference of levels if any in the canal bed levels on the U/S and D/S side of the regulator.

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Q = C Bt H3/2 WhereQ = D/S full supply discharge in m3 /sec C = Co-efficient of discharge Bt = Clear water way in metres. H = Head over crest i.e. Full supply level on the U/S + head due to velocity of approach – crest level. The value of ‘C’ is determined using Malikpur graph (a graph drawn between drowning ratio and co-efficient of discharge based on experiments).

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Crest level is calculated as per CWC, Manual i.e, Crest level = U/s TEL – head over crest (H) The height of crest above up stream bed level should not be more than 0.4 H. Glacis profile is calculated as per CWC manual with 2:1 slopes to negotiate the levels and smooth curves at the junctions. The radius of curvature to be adopted is H/2 on up steam and ‘H’ on downstream as specified therein.

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D/s floor of the regulator is depressed to form a cistern to dissipate energy. Since the U/s and D/s C.B.Ls and F.S.Ls are almost the same in the NSP Canals and distributaries, the energy dissipation arrangement is quite simple. To dissipate energy at low flows through regulator the cistern with water cushion with a minimum length and deflector wall at the end of the cistern are provided. On main system the hydraulic jump calculations are to be done for different opening conditions i.e., ¼, ½, ¾ and full supply. Further if there are more than one vent, these calculations have to be made for different conditions of vents opening. The height and length of jump in each case is to be found. Based on these calculations the depth and length of cistern will be fixed. Refer I.S:4997–1968 or Small Dams by USBR.

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When there is no water on D/s of the regulator and water at FSL on U/s, the exit gradient is to be calculated and the thickness of floor has to be designed for the uplift pressures at various sections. The formula for exist gradient is: GE = 1x H ( π √ λ ) d Where: λ = α 2 2

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2) Crest Level: The Crest level is fixed by working out ‘D’ using formulae Q - = C. Bt. D 3/2 Where Q = discharge in cumec C = co-efficient of discharge depending on the drowning ratio. Up to 70% fluming C = 1.84 can be adopted and above that, it is to be read from Malikpur graph Bt = Throat width in ‘m’. D = Depth of crest below U/S TEL in ‘m’ After calculating value of D from the formula, crest level is fixed with the equation: Crest level = U/S TEL – D

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3) Length of Crest: 2/3x D. 4)Height of Crest: Should not be greater than 0.4 D, above the upstream canal bed level. 5)D/S Glacis: In the case of baffle type glacis drops, glacis slope is to be 2/3: 1 joined tangentially to the crest on the U/S side and baffle platform on the downstream side with radius equal to ‘D’. In the case of straight glacis provide glacis slope of 2:1 with radius of curvature as D at the junction with the crest at the upstream end and pavement at the downstream end. 6)U/S Glacis: Glacis slope is to be ½: 1 joined tangentially to the crest with a radius equal to D/2.

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7)Protection: (i) Length of U/S protection: 3 times F.S.D. or as per the standard fixed by the project authority. The protection is in CC M 15 grade with profile walls at the end. (ii) Length of D/S protection: 4 (d + h) where d = d/s F.S.D. and h = difference in F.S.Ls or as per the standard fixed by the project authority. The protection is in CC M 15 grade with profile walls at the end. 8)Glacis fall without baffle: (i) The hydraulic jump is calculated to be the most efficient means of dissipating the energy. To ensure formation of the hydraulic jump, it is necessary that the depth of tail water flowing at sub–critical velocity in the canal downstream should bear the following relation to hypercritical depth of flow at the toe of glacis:

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ii)Cistern: The cistern level is obtained by subtracting the value of 1.25 dx or 1.25 dx1, as the case may be, from the downstream full supply level of the canal or 1.25 Ef2 from the downstream total energy level, which ever gives the lower level. Ef2 is the energy of flow in the canal after formation of the hydraulic jump. The length of the cistern is equal to 5 Ef2. The cistern is joined to the downstream bed at a slope of 1 in 5. 9) Glacis fall with baffle: The dimensions of the baffle platform and baffle wall are determined from the relationship given below: (i) R.L of Baffle platform: D/S F.S.L. – d1x. (ii) Height of Baffle wall (Hb) = dc – d 2 Where, d 2 = Hyper - critical depth at the point of formation of standing wave.

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v) Cistern: (a) Depth of cistern: D/S FSD/10 subject to a min of 15 cm for distributaries and minors and 30 cm for main canals and branches. (b) R.L. of the cistern = D/S bed level – depth of cistern (c) Length of cistern = 5 times down stream F.S.D. (d) R.L. of the deflector wall = D/S CBL + D/S F.S.D/ 10 10) Friction blocks and glacis blocks: (i) Glacis fall with baffle (a) If the height of drop is less than 2.0 meters, friction blocks and glacis blocks are not required. If the height of drop is more than 2.0 m, two rows of friction blocks staggered in plan are to be provided.

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Size of friction blocks: Height (h) = dx, Length (L) = h Top width (W) = 2h / 3 Distance between two rows = h. The downstream edge of downstream row of friction blocks shall be provided at a distance of one third length of cistern from the end of the cistern floor. b) Glacis blocks: Single row of glacis blocks of same size as friction blocks is to be provided at the toe of the glacis. (ii) Glacis fall without baffle

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Four rows of friction blocks staggered in plan are to be provided in the case of flumed falls. The upstream edge of first row of blocks may be at a distance of 5 times the height of blocks from the toe of glacis. Size of friction blocks: Height (h) = D/S FSD 8 Height (L) = 3h Height (W) = 2h 3 Distance between rows = 2h 3 11)Deflector wall: In glacis falls, a deflector wall of height equal to one tenth of the downstream FSD is provided at the downstream end of the cistern. The minimum height should be 15 cm.

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After working out values of H/d and b/d, find the value of exit gradient GE from the graph in plate 16 of CWC manual on falls. The GE depends upon the soils, but it should be less than Uplift Pressure: (a) U/S curtain wall: 1 = d = depth of D/S curtain wall α blength of impervious floor Find out corresponding value of φ E = from graph i.e., from plate 17 of CWC manual on falls. … % of residual head φ E1 = φ E b) At the d/s cut off wall 1 = d = depth of D/S curtain wall α b length of impervious floor

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Find out the corresponding value of φ E from graph i.e, from plate 17 CWC manual on falls. ii) Thickness of floor: The uplift pressures at toe of glacis, at the end of baffle and at the end of cistern are worked out by interpolation for fixing the thickness of floor. Thickness of floor at toe glacis: % age of toe of glacis = φ E at D/s + (φ E1 − φ E D/s) X L/b b = total length of impervious floor. L = Length of floor up to toe of glacis from D/S end. Thickness of floor at the toe of glacis = %age of toe of glacis x H 100 x (ρ − 1) Where ρ is specific gravity of CC i.e., 2.4

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Similar method is to be adopted for calculating thickness of floor at the end of the baffle, at the end of cistern etc. FIG.17

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At the end of floor 1=d α b. φ E is read from ‘Plate 11.1 (a) of CWC manual on fall (enclosed) Thickness of Floor at the d/s Face of drop wall is interpolated considering the pressures at the face of crest wall and at the end of floor. Absolute pressure = (% Pressure) x H m of water column. 100  P = 75% of Absolute pressure for soils other than pervious soils

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Thickness of floor = P, where ρ = 2.40 ρ – 1 c) Friction Blocks: For discharge exceeding 3 cumec, two rows of friction blocks staggered in plan may be provided in cistern. The downstream edge of downstream row should be at a distance of one third the length of the cistern from the downstream end of cistern floor. Size of friction blocks: Length (L) = 1 x Downstream F.S.D. 8 Height (h) = 1 x Downstream F.S.D. 8

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Top width (w) = 1 x height of subject to minimum of 8 cm, joined to floor on the 4 downstream side with a slope of 1:1 Clear space between rows = height of the blocks. Vertical type core wall drop: (CE NSLC Circular No. DW.150/ 3845 – S, ) Various components of the vertical type drop with core wall for different ranges of discharges i.e., 1.5 cumec to 1 cumec, 1 cumec to 0.5 cumec, 0.5 cumec to 0.1 cumec, 0.1 cumec and below and for various heights of drops i.e., 0.6 m, 0.8 m, 1.0 m, 1.2 m and 1.5 m with clear over fall are given in table I and II. The same may be adopted for the drops on the distributories having discharge 1.5 cumec and below.

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For drops in silty or clayey soils the following modifications may be adopted (Design Circular No. 35/1807 dated of C.E., N.S.L. Canals). (a) For drops of 1.5 m and above, for all discharges, wings and returns may be provided. (b)For drops less than 1.5 m height and discharge above 1 cumec, wings and returns may be provided. Following are the recommendations of the Expert Committees on design of drops on distributary system.

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(a) For drops with height of less than or equal to 0.60 m and discharge of less than 50 cusec, unflumed core wall type drops may be provided. (b) For drops with height more than 0.60 m and discharge between 50 and 100 cusec, unflumed vertical drops with wings and returns may be provided. (c) For drops with discharges more than 100 cusec, straight flumed drops may be provided. Where fluming ratio as per codel provision could not be adopted for drops of height less than 0.60 m, unflumed vertical or unflumed core wall type drop may be provided.

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C = The coefficient of discharge of notch = 0.70 d = depth of water in metres over sill of the not L = width of the horizontal sill of the notch in ‘m’. n = 2 tan α, where α is the angle made by each of the sides of the notch with the vertical. If ‘n’ is Zero, then it becomes a rectangular notch. Case II: For submerged notch: Q= 2.96 C√ d-E E +d L + 3 E 2 + (d-E) E (d-E) 2 n 2 4

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Where E = the submersion depth of tail water over the sill of the notch. Q, C, d, L, n are the same as in the case – I. Find L and n by using the above equations (free fall or submerged) for full supply discharge and half supply discharge conditions. Substitute the values of L and n to get top width of notch in the equation = L + nd. 4)Length of drop wall between abutments: Length of drop wall between the abutments should not be less than 7/8 th of the canal bed width on up stream. However in practice, the length of drop wall is provided equal to upstream bed width.

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(5) Width of notch pier at FSL should not be less than half of upstream F.S.D. ‘d’ Top width of notch is generally 0.75 d, where notch is free and d where notch is submerged. 6)Water cushion: The depth ‘x’ of the water cushion is worked out from the following equation X + d1 = 0.91 dc √ h Where d1 = D/S F.S.D dc = Depth of water over the crest. h = height of drop (difference in FSLs).

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7)Length of cistern: Length of the horizontal floor of the cushion = 2 dc + 2 √ dc h subject to a minimum of √dc h. It is to be designed on the basis of up lift pressures and exit gradient if the soil is pervious. 8)Thickness of cistern floor = 0.55√ dc + h. This should be designed on the basis of uplift pressure and exit gradient, of the soil is pervious.

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Trapezoidal notch core wall drops :( CE NSLC Circular No. DW.150/ 3845 – S, ) Various components of the notch type drop with core wall for different ranges of discharges i.e., 1.5 cumec to 1 cumec, 1 cumec to 0.5 cumec, 0.5 cumec to 0.1 cumec, 0.1 cumec and below and for various heights of drops i.e., 0.6 m, 0.8 m, 1.0 m, 1.2 m and 1.5 m with clear over fall are given in table I and II. The same may be adopted for drops on distributaries' having discharge of 1.5 cumec and less. For drops in silty or clayey soils the following modifications may be adopted.

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i). For drops of 1.5 m and above, for all discharges, wings and returns may be provided. (ii). For drops less than 1.5 m height and discharge above 1 cumec, wings and returns may be provided. TABLE No. I (A) DETAILS OF COMPONENTS OF NOTCH TYPE DROPS WITH CORE WALL (FREE FALL) FOR DISCHARGES LESS THAN 1.5 CUMEC AND HEIGHT OF DROP LESS THAN 1.5 m

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D/S Discharge in Cum Height of drop h in m No. of notches Details of each notch X cushion Thickness of end pier Top of drop wall Lt Bottom width of drop wall Length of drop core wall Length of apron (La) Thickness of apron (ta) LL + nd to Refer Table II (A) to to and below

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TABLE No. II (A) Table showing width of Notches at sill and at top of various discharges for Notch type drop with free fall. Discharge through each notch Q (cumec) is given by Q = d 3/2 (L+ 0.4 nd) Where d = Depth of flow over sill (metres) L = width of notch at sill in metres n = 2 tan θ where θ is the angle made by each of the sides of the notch with the vertical. Top width of notch at F.S.L = L + nd.

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Notch type drop with core wall : In core wall type, the drop wall is combined with a straight wall, which is extended into the banks with proper keying. There are no wings & returns on the U/S and D/S sides. But CC apron and side protection with CC lining (better if double the normal thickness provided) is provided. i)Formulae adopted for working out the rectangular notch Q = [ L – 0.1 nd] d 3/2 Where n = no. of notches

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L = length of the drop wall in metres d = depth of water in meters over the crest of drop ii)Formulae adopted for trapezoidal notch is same as discussed in the previous case. iii) Length of apron, thickness of apron and water cushion – same as discussed in the previous case (trapezoidal notch).  The drops can be combined with bridges wherever possible. In such cases the clearance between sill of drop to deck may be provided as below:

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OFF TAKE SLUICE: Off takes are provided on the conveyance system to irrigate the ayacut localized under branch or distributary. As per World Bank norms, the water distribution system is broadly classified as: i)Supply system or conveyance system. ii)Distributary System. 1) Supply system or conveyance system: Main canal, branch canals and majors carrying a discharge above 5.66 cumec (200 cusecs) are considered as supply system. They will run continuously. The distributaries taking off from these have gated structures if the carrying capacity is 5.66 cumec (200 cusec) and above.

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2) Distributary system:  The distributaries have capacity less than 5.66 cumec (200 cusecs). These will run either full or closed. The water will be distributed proportionally through modules (APM or OFM). No gated structures will be there on the distributary system.  In the first reach of distributary, a standing wave flume which is used as a measuring device, is provided. Gated off – takes:  These may be either:  i)Rectangular/ square vents covered with R.C.C slab or  ii)Pipes

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 Rectangular vents:  (1) Sill level: The sill of O.T is kept either at or above the CBL of parent canal depending on the ratio of discharges in distributary and parent canal. % of O.T. discharge to parent canal discharge Height of sill of sluice above the CBL of parent canal when FSD in the parent canal is: Above 2.14 m2.14 to 1.22 mBelow 1.22 m 15% and above % to 15% % to 10% m 2% to 5% m 2% and less m

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(2) Driving head: (3) The driving head at O.T. is arrived at normally considering half supply discharge in the parent canal when the full supply discharge flows into the distributary channel.  Driving head = Supply level in parent canal for half supply discharge FSL in distributary  The FSL of off take channel is generally fixed at 15 cm below the half supply level of parent canal for the channels taking off from main canal and branch canal and 7.5 cm for channels taking off from the distributaries. However vent way is designed with minimum driving head of 7.5 cm (3”) for pipes. The level difference between the sill level and C.B.L. of parent canal is negotiated by proving suitable longitudinal slope.

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 The length of barrel is worked out with respect to the position of D/S head wall. The flow condition in the barrel is dependent on D/S condition in the O.T. channel immediately below the vent way. TELs at entrance and exit of barrel are calculated and checked for assumed level.  4) R.C.C. slab under head wall:  It is designed to withstand for the max stress at the bottom of head wall (resting over the slab) in addition to its self weight. The slab is constructed in VRCC M 20 grade with HYSD bars.

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(5) R.C.C. slab under earth bank: It is designed for weight of earth over it in addition to its self weight. Live load is also to be taken into consideration for the slab under inspection path. (6) Transitions: The U/S and D/S transitions are provided with 1 in 3 and 1 in 5 splay respectively as per practice.

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(7) Stilling Basin:  The type of stilling basin to be provided depends upon the velocity at the entry of barrel. If the entry velocity is above 6.1 m/sec. (20 ft/sec) the barrel floor is depressed both for rectangular and square vents based on the hydraulic jump calculations. In case of normal velocities which are of the order of 4 m/ sec the floor is at the same level and the floor is checked for arch action for the uplift pressure.  The design of hydraulic jump basin for energy dissipating arrangements can be followed from ‘Small Dams’ by U.S.B.R. or as per I.S – For shooting flows, an impact type basin - VI with R.C.C. baffle wall is to be provided. The baffle wall is to be designed for the maximum water thrust with 50% impact factor when full discharge is let out in the canal.

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(2) Laying of O.T. Pipes:  The condition of laying of off – take pipes such as “Negative projecting condition”, and “Trench condition” etc., relevant to the individual cases are followed as per IS. 783 – 1985 for laying and jointing. For the selection of proper size of pipe for the vents, IS. 458 is to be followed.  Controlling arrangements  The following controlling arrangements are followed.  Type of control to be adopted:

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i) For pipe sluices of 6” (150 mm) dia and below and vents of  equivalent area with F.S.D of parent canal not exceeding No control  4ft (1.22 m). and O.T discharge 1.5 c/s and less (ii) For pipe sluices of diameters above 6” and upto and including 12”(300 mm) with F.S.D of parent canal not Stem shutter exceeding 4ft (1.22 m). iii)For all sluices where the FSD in the parent canal Screw is more than 4ft (1.22 m) and for sluices of larger ventways. gearing shutter

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SEMI MODULAR OUTLETS The Expert Committee (Core Committee) suggested to provide Semi modular outlets (ungated ) for the outlets with discharge of 0.5 cumec and less, taking off from channels having discharge less than 25 cusec (about 0.7 cumec) Definition of semi modular outlets (flexible modules) The outlets whose discharge is independent of the water level of the outlet channel but depends on the water level of the distributary so long as minimum working head required for their working is available. The discharge through such an outlet will therefore, increase with the rise in the distributary water surface level and vice versa. The common examples of this type of modules are

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(2) Adjustable Orifice Semi - Module (A.O.S.M):  An adjustable orifice semi - module consists of an Orifice provided with gradually expanding flume on the d/s side of the orifice. The flow through the orifice is super critical, resulting in the formation of hydraulic jump in the expanded flume portion. The formation of jump makes the discharge independent of water level in the out let channel. 3) Adjustable Proportional Module (A.P.M): This type is the most commonly used outlet in this class. In this, the CI roof block is fixed to the check plates by blots, which can be removed and depth of outlet adjusted after masonry around is dismantled

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 (4) Pipe Semi - Module (P.S.M):  Pipe outlet discharging freely into atmosphere is the simplest and the oldest type of flexible outlet. The discharge through such an outlet will depend only upon the water level of the distributary and will be independent of water level in the outlet channel so long as the pipe is discharging freely. This can be provided where sufficient level difference between distributary and outlet channel is available.]  The suitability of the type of the semi module outlet is determined based on the ratio of parent canal discharge (Q) to the discharge of the out let (q) and the throat width (Bt) as detailed below.

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The above conditions are further explained as below Arrive at the ratio of parent channel / out let channel. If it is < or = 20, select OFM. Calculate the Bt ( throat width ), using weir formula. If Bt is > 6 cm it is ok. Otherwise select A.P.M. Work out the Bt using the sluice formula setting the crest of outlet at less than 0.80 D from FSL of Parent Channel and adjusting the height of outlet opening. If Bt = or > 6 it is ok

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 Otherwise go for pipe semi module (PSM), if it is possible to do so. Check for proportionally Open flume module Discharge through the out let (q) in cumec is given by the formula:

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Pipe semi module Design criteria The discharge through pipe semi module is given by Q =Cd. A (2g h c ) 1/2 Where Cd = 0.62 for free pipe out let h c =head on U/S above the centre of pipe h c should be more than 1.5 times the dia of the pipe proposed. The above formulae can be reduced to Q =0.62 x √ (2x 9.81 ) A √ (hcnt)  =2.746 A h c 1/2

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For free fall condition set the F.S.L of OT Channel below the pipe sill level keeping in view the command under the pipe sluice.It is a simplest type and the users will appreciate. Throttling the vent way of existing pipe out lets: (From design guidelines for structured irrigation network to suit to RWSS). When the existing diametre of pipe is more than required then, to reduce the size of the pipe a sleeve pipe is introduced whose diametre is worked out by equating operating head to the headloss.

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FLOW MEASUREMENT STRUCTURES GENERAL Provision of "measuring structures/devices" shall conform to the following guide lines given by Sri R.K. Malhotra, World Bank Consultant.  A measuring structure is to be provided downstream of every off-take of major from the main canal/ branch canal, distributory from a major, minor from the distributory and sub-minor from the minor etc. Measuring structure is also to be provided at off-take of branch canal from the main canal and also in the main canals.

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 Types of measuring structures shall be broadly: "Standing Wave Flumes” in concrete (SWF) and Parshall Flumes & Cut Throat Flumes (CTF) in fiber glass reinforced plastic material with their hold-fasts to be embedded in concrete structures. Standing Wave Flumes may be provided in the main & branch canals; Cut Throat Flumes /Parshall Flumes in the majors/distributaries, while Cut Throat Flumes may be provided in the minors/sub-minors. The Parshall and Cut Throat Flumes in fiber glass reinforced plastic (FRP) material shall have engraved gauge markings in centimeters as well as in liter/second.  Division Boxes shall be constructed in concrete. Likewise, turn-outs shall be constructed in concrete.

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 STANDING WAVE FLUME  Standing wave flume is a critical depth flume. The discharge through this is independent of water level on downstream and varies with water levels on upstream. The hydraulic behavior is same as that of a broad crested weir. Since only one gauge reading is required to be taken for measuring the discharge and due to ease of construction, standing wave flumes are recommended as a flow measuring device.  The following are the three types of flumes proposed for adoption

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1. Standing wave flume 2. Standing wave flume fall (associated with drop) 3. Rectangular throat flume (to be adopted on canals having discharge less than 1 cumec) Design criteria The design is as per IS "Method of measurement of flow of water in open channels using standing wave flume”

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For Q Value of C f 0.3 to 1.5 cumec to 15 cumec0.99 above 15 cumec 1.00 Modular Limitvalue of submergence ratio of H2/H1 at which the real discharge deviates by 1 % of Q calculated by discharge equation. It should be between 0.7 to 0.95 With straight transition from throat width to downstream bed width in a length of 4 H Modular Limit H2 /H1 = 0.8 to 0.85 Minimum modular head =0.15 H to 0.2 H

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 2) Height of hump :  The height of hump is the difference between the u/s canal bed level and the sill level of the flume. Height of hump, for proportionality between full supply and any fraction of full supply between the channel and weir is given by the equations. 1 m 1/x 1 m 2/3 (i) Z = d 1 – D 1 = d 1 m 1/x 1- For channels running with fluctuating discharge

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 where m = Q m = Any particular fraction of full supply discharge Q x =approach channel index d 1 =upstream depth of water in the canal D 1 =Depth of water over the crest Where Q = discharge C 1 = a coefficient d 1 = depth of water in the channel x = index, which varies from 1.5 to 2

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 From the discharges Q,Q',Q'',Q''', etc, for the flow of depths of d1,d'1, d''1,d'''1, etc respectively, the value of x in the equation is estimated by least square method by considering 4 sets of d and corresponding Q. ∑ log Q. Log d - (∑ log Q ) ( ∑ log d) Where M = No. of sets = 4 x = M  ∑ ( log d ) 2 - ( ∑ log d ) 2 M Figure 2 gives the height of hump required for various values of x and fluctuations. In case of channels which run either full or closed, a flume which gives proportionality at full supply discharge is desirable. In the case of channels, in which discharge varies considerably, bulk proportionality is preferable. Figure 3 gives the heights of hump for bulk proportionally.

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(3) Head loss: The head loss consists of the following losses: (i) Approach transition, (ii) Exit transition, (iii) Friction in structure, and (iv) Hydraulic jump The loss in approach and exit transitions depends on the amount of fluming and its gradualness. The friction loss is usually very small. The loss in hydraulic jump is given by the equation: H L = (d 2 – d 1 ) 2 4 d 1 d 2 Where d 1 = depth of flow before jump d 2 = depth of flow after jump

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(4) Approach transition The radius of side walls of the bell mouth entrance should be 3.6 H 1.5 metres. If ‘H’ is less than 0.30m, the radius may be 2H from the throat. The curvature (formed from the throat) should continue till it subtends an angle of 60 0, from where, it should be continued tangentially to meet the side of the channel upstream. The bed convergence should begin on the same cross section as the side convergence. The radius of curvature of hump in the bed should be: r h = L Z 2 2 Z

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Where r h = radius of curvature of hump L 1 = length between the junction of side wall with the bed of upstream channel and upstream end of the throat measured along the axis. Z = height of hump above u/s bed level.

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FIG.42

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FIG.43

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5) Throat Sides of throat should be vertical and length should be 2.5 H. Width of the throat way be calculated from the formula given in sub-para (1) of (6) Downstream glacis The length of downstream glacis should be equal to 4H, which is also the length of the side walls along the glacis. The slope of the glacis is usually 1 in 20 or flatter. The divergence of side walls should be 1 in 10 or flatter so as to make the width at the toe of the glacis equal to or less than the downstream canal bed width.

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(7) Gauge (Stilling) well The stilling well should be so located as to measure the water upstream of the sill, where there is no curvature of flow. This could be ensured by locating the stilling well intake pipe at a distance of 4 H max upstream of the bell mouth entrance. H max is the maximum value of upstream head over the sill (including velocity head).

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STANDING WAVE FLUME FALL (ASSOCIATED WITH A DROP) Standing wave flume fall is a flow measuring device.It acts as a control point to maintain design supply level in the canal on u/s of the structure. A measuring device to be provided at the head of the distributory shall be a standing wave flume combined with fall if it exists at reasonable distance from head before first drawal. In case any existing drop is damaged, requires reconstruction and satisfies the above condition, it can be reconstructed with standing wave flume. The drops downstream of the outlets may be designed as the standing wave flume fall, wherever necessary.

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 Design criteria The standing wave flume fall is essentially a broad crested weir and IS: "Method of measurement of flow of open channels using standing wave flume fall" and "Manual on canal falls" by Central Water Commission are followed for the design of standing wave flume fall. The design calculations are similar to that of standing wave flume. The main difference between the two is in the energy dissipation arrangements. In the case of normal standing wave flume, head loss is considerably low and does not require any special energy dissipation arrangements. In the case of standing wave flume combined with fall or drop, energy dissipation arrangements are provided as per the requirements for the falls.

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(1) Discharge Discharge through standing wave flume ( Q ) in cumec is given by the equation given in sub- para (1) of para 6.1.1, similar to that for standing wave flume without fall. In case, piers are provided in the flume, the discharge is given by the formaula: Q = 2 2 g. C f (B o – mb – 2C c m H) H √ 3 Where Q = discharge in cumec  C f = Coefficient of friction having the following values:

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0.97 for Q = 0.05 to 0.30 cumec 0.98 for Q = 0.30 to 1.50 cumec 0.99 for Q = 1.50 to 15.0 cumec 1.00 for Q = 15.0 cumec and above B o = Overall throat width including piers m= no. of piers b= thickness of each pier C c = coefficient of contraction having values of for piers with round nose and 0.04 for piers with pointed nose. H= head over sill including velocity head given by equation

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H = D 1 + V a Where D 1 = upstream depth of water over sill, and V a = velocity of approach (2) Height of hump: (3) Throat : The length of throat is equal to 2.5 H. The throat width is calculated from the discharge formula in sub-para (1) of para The width of throat shall not be less than 1.5 H

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(4) Inlet transition: The radius (R) of the side walls of bell mouth entrance should be 3.6 H 1.5. The curvature should continue till it subtends an angle of 60 0, from where it shall be continued tangentially to meet the side of the channel. However when the curved walls meet the sides of channel when it subtends an angle of 60 0, it is not necessary to continue the walls further. The length of inlet transition (L 1 ) may be found out knowing B 1, B 2 and the radius of bell mouth entrance R using the relation: L 1 = 2 R - B 1 – B 0 B 1 – B 0  √ 22

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Where B 1 = upstream bed width of channel B 0 = overall throat width The radius of curvature of hump (r h ) in the bed is given by the following equation. (r h ) = L Z 2 2 Z When the total head above the standing wave fall (SWF) sill becomes considerable, say 1.2 m, the height of hump ‘Z’ becomes insignificant as compared to ‘L 1 ’ so that the radius becomes large and the U/S end of the throat may be joined by a straight line to the channel bed U/S.

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5) Design of Glacis : The glacis should have a slope of 2:1 connected with the throat upstream by a curve of radius 2H and with the cistern downstream by a curve of radius H. The side walls should be straight over glacis portion. With steeper glacis slope of 2:1 and greater loss of head, proper expansion should be provided. For controlling the issuing flow, parallel sides should be extended down to the toe of glacis followed by hyperbolic expansion in the cistern using equation: B y B 0 B 3 L L B 3 - ( B 3 - B o ) y

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Where, B y = width at any distance ‘y’ Y = distance from beging of expansion of hyperbola, B 0 = over all throat width of flume at the contracted section (excluding piers) B 3 = bed width of downstream channel L = length of cistern (6) Cistern The cistern is provided at the toe of the glacis. The length of cistern (L) varies from 4 times to 6 times the downstream FSD (d 3 ) of the channel depending on the nature of soil in the channel bed.

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4 d 3 for shingle bed L = 5 d 3 for good earthen bed 6 d 3 for sandy bed If the channel is lined with CC, length of cistern may be taken as 4 d 3. In order to stabilize the flow, bed of cistern should be made steeper in the center by 25% compared to the sides. (7) Control blocks Two rows of control blocks, staggered in plan should be provided downstream of the toe of the glacis in the cistern. The size of the blocks should be as follows:

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Height (h) = 1/6 depth of water in mid cistern Length (l) = 1.5 to 3 h Width (w) = 2/3 h Clear distance between blocks = l Clear distance between rows = w The first row of blocks should be at 3 to 5 times the height of the blocks from the toe of glacis. 8) Deflector A deflector should be provided at the downstream end of the cistern.

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Size of deflector: Height (h) = 1/12 depth of water in mid cistern Width (w) = h Gap in the deflector = h Internal of gaps = 4h Short walls of same height should be placed close to the upstream of gaps. (9) Gauge well

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RECTANGULAR THROAT FLUME: The discharge in a open channel may be measured by means of a flume. Consisting essentially of contractions in the sides and / or bottom of the channel forming throat. When the dimensions are such that critical flow occurs in the downstream, (in other words it is free flowing) discharge can be determined from the single upstream depth measurement. This device is called “Critical Depth Measuring Flume ".This structure may be adopted for measuring smaller discharges less than 1 cumec.

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Design criteria: (a) Rectangular throat with hump (b) Let 'Y' be the depth of flow and velocity be "V" m/sec in the normal section. Then total energy head is equal to depth of flow and due to velocity of approach i.e E = Y + V 2 / 2g Take the value of 2 g equal to 19.2 In Rectangular section, critical depth (Y c ) is equal to two thirds the Total Energy head,i.e Y c = 2/3 E